1. Field of the Invention
The present invention relates to an object (a product including a machine, a manufacture, and a composition of matter) and a method (a process including a simple method and a production method). In particular, one embodiment of the present invention relates to a nonaqueous secondary battery, or relates to a method for restoring a capacity of a nonaqueous secondary battery.
2. Description of the Related Art
Lithium-ion secondary batteries have advantageous effects such as high output and high energy density and have been frequently used for a variety of uses such as portable electronic devices, next-generation clean energy vehicles including hybrid electric vehicles (HEVs) and electric vehicles (EVs), and stationary power storage devices. In particular, development of comparatively large-sized secondary batteries for vehicles and stationary power storage devices, which can be used for a long period of time, has been needed along with a growing demand for energy saving in recent years.
A lithium-ion secondary battery, which is one of nonaqueous secondary batteries, includes a positive electrode, a negative electrode, a separator, a nonaqueous electrolyte solution, and an exterior body covering these components. In lithium-ion secondary batteries, positive electrodes and negative electrodes are generally used; the positive electrodes each include a positive electrode current collector made of aluminum or the like and a positive electrode mix which includes a positive electrode active material capable of occluding and releasing lithium ions and which is applied to both surfaces of the positive electrode current collector, and the negative electrodes each include a negative electrode current collector made of copper or the like and a negative electrode mix which includes a negative electrode active material capable of occluding and releasing lithium ions and which is applied to both surfaces of the negative electrode current collector. These positive and negative electrodes are insulated from each other by a separator provided therebetween, and the positive electrode and the negative electrode are electrically connected to a positive electrode terminal and a negative electrode terminal, respectively, which are provided for the exterior body. The exterior body has a certain shape such as a cylindrical shape or a rectangular shape.
When a secondary battery is used for a long period of time, e.g., for several years or for more than ten years or more, a problem of a decrease in capacity arises. A cause of the decrease in the capacity of a lithium-ion secondary battery is a reduction in lithium ions contributing to a battery reaction.
There are several causes of the reduction in lithium ions contributing to a battery reaction. One of them is formation of a coating film on a surface of a negative electrode. Specifically, an electrolyte solution is decomposed at the interface between the negative electrode and the electrolyte solution, and a coating film containing lithium is formed on a surface of the negative electrode. Formation of the coating film allows a stable battery reaction; however, excessive formation of the coating film is not preferable because lithium ions contributing to a battery reaction are reduced.
The following is another cause of the reduction in lithium ions contributing to a battery reaction. By rapid charging or the like, metal lithium is deposited on an active material and a surface of an active material and separated from a current collector.
In normal low-rate charging, metal lithium is deposited on an active material and a surface of an active material. In the case where conductivity is kept between the active material and the metal lithium, the deposited metal lithium slowly disappears in discharging. However, the metal lithium expands and shrinks when the active material, particularly the negative electrode active material, occludes and releases a lithium ion. Therefore, when rapid charging is performed or deposition of metal lithium is repeatedly performed for a long period of time, the deposited metal lithium loses its conductivity with the current collector, leading to separation of the metal lithium in some cases.
The separated active material or metal lithium causes clogging of the separator, whereby the diffusibility of lithium ions is reduced and lithium ions are concentrated in the periphery of the clogging, leading to further deposition of metal lithium and a further decrease in the capacity of the secondary battery.
Another cause of the decrease in the capacity of a lithium-ion secondary battery is deterioration of an electrolyte solution. An electrolyte solution which normally functions is reduced by decomposition of the electrolyte solution at the interface between an electrode and the electrolyte solution, vaporization of the electrolyte solution due to heat generated in a battery reaction, and the like, reducing the diffusibility of lithium ions.
The reduction in the diffusibility of lithium ions causes further deposition of metal lithium as described above, leading to a vicious cycle.
In order to suppress the above-described reduction in lithium ions contributing to a battery reaction, various methods have been studied. For example, a method for predoping a negative electrode with lithium before assembly of a secondary battery is known. As another method, a metal lithium electrode is provided in advance as a third electrode inside a secondary battery, and lithium ions are supplied from the metal lithium electrode when its capacity is decreased (Patent Document 1). Furthermore, in another method under study, a metal lithium electrode which has been installed in a cassette case is inserted into a secondary battery to supply lithium ions when its capacity is decreased (Patent Document 2).